Organic matter is the vast array of carbon compounds in soil. It is originally created by plants, microbes and other organisms. These compounds play a variety of roles in nutrient, water, and biological cycles. Organic matter covers a wide range of things, such as lawn clippings, leaves, stems, branches, moss, algae and lichens, parts of animals, manure droppings, sewage sludge, sawdust, insects, earthworms and microbes. Organic matter increases the nutrient holding capacity of soil. It serves as a pool of nutrients for plants and chelates nutrients, preventing the nutrients from becoming permanently unavailable to the plants. It is food for soil organisms, which hold onto the nutrients and release them in a form that is readily available to plants. Additionally, organic matter improves the water dynamics of soil and affects soil structure by reducing crusting and encouraging root development and aggregation.
The loss of organic matter from soil, also known as soil depletion, occurs when the components that contribute to fertility are removed and not replaced, and the conditions that support soil fertility are not maintained. Soil depletion can then lead to poor crop yields. In agriculture, for example, depletion can be due to excessively intense cultivation and inadequate soil management. Depletion may also occur through a variety of other effects, including overtillage (which damages soil structure), overuse of inputs, such as synthetic fertilizers and herbicides (which leave residues and buildups that inhibit microorganisms), and salinization of soil.